In the forestry industry, it is common to use a vehicle known as a log skidder to transport fallen trees from one location to another, such as from the forest to a processing plant or loading area. Some of the log skidding vehicles utilize a grapple to grasp one or several trees by one end, lift that end from the ground and drive off dragging the trees behind the vehicle. The grapple includes a pair of mechanically actuated tongs that may be moved towards and away from each other to grasp and release the trees. The grapple frame or head is mounted to a boom that extends from the rear frame of a vehicle and may be raised and lowered to adjust the height of the grapple. A rotator is very often mounted within the boom frame and is selectively rotated by a hydraulic motor or equivalent means to aid in positioning the grapple tongs with respect to the desired load. A swivel link is normally positioned between the grapple head and the rotator and allows relative rotation therebetween about two axes that are oriented 90 degrees from each other.
While these types of log skidders have been known to work quite well for engaging and transporting logs, uncontrolled swinging of the grapple head has presented a common problem when the vehicle is driven without a load. Because of the multiplicity of pin joints that allow the grapple head the needed versatility when engaging a log, the entire apparatus tends to swing at will when driven unloaded over rough terrain. Often during this swinging, the grapple fame or tongs are brought into rather severe contact with the surrounding vehicle structure causing damage to any one of numerous components.
In order to resist this uncontrolled swinging movement, several types of motion dampers, or snubbers have been included in the area of the pin mountings between the swivel link and its connections to both the boom (or rotator) and the grapple head. One such device is the subject of a Product Information Catalog 275-1A published by the Esco Corporation and entitled, "Type III Snubber Assembly".
In that device, a friction disc retainer is positioned between the mounting flanges of the boom and each opposing face of the swivel link member. The friction disc retainer mounts replaceable friction discs for contact with the faces of the swivel link. The discs are urged into contact with the swivel link by a belleville spring that is positioned between the friction disc retainers and the boom mounting flanges. A mounting pin is centrally disposed through these members and has a threaded end portion that mounts a nut that clamps against the outside surface of one of the boom mounting flanges while a locking pin fixes the position of the mounting pin with respect to the opposing boom mounting flange. The nut may be turned inwardly, drawing the mechanism together to increase the force applied by the belleville springs that urges the friction discs against the swivel link. This increases the amount of resistance to relative movement between the two members. A similar arrangement is utilized between the swivel link and the grapple head. A major drawback with an arrangement of this type resides in the lack of protection that is afforded the snubbing mechanism from exposure to the elements and/or oil and grease that are used to operate and lubricate surrounding components. When any of the above, especially oil or grease, find their way into and around the area of the friction discs, the effectiveness is greatly reduced. Also a design of this type requires a great deal of readjustment and maintenance of the friction discs. In this particular design, the nut on the end of the mounting pin is so large than an equally large adjustment wrench must be utilized to provide sufficient force to turn the adjusting nut. Further, the swivel link cannot be disconnected from either the boom or the grapple head without complete disassembly of the snubber mechanism.
Another rotation dampening arrangement is disclosed in U.S. Pat. No. 4,335,914, issued to Adrian C. Korbel on June 22, 1982. This device utilizes one of the mounting pins that mounts the swivel link to either the rotator or grapple head as part of the dampening mechanism. The pin includes an enlarged head portion that extends outwardly into a housing that is formed in part by the swivel link. A plurality of brake discs extend between the housing and the pin head and are splined to each member. The major drawback with an arrangement of this type is the outward location of the damper housing with respect to the swivel link. Being so positioned, it is exposed to contact with various surrounding vehicular components as well as objects that may be encountered in the rugged environments in which it operates. Also, complete disassembly of the dampening mehanism is required when separation of the swivel link from either the boom rotator or grapple head is desired.
Yet another motion dampening mechanism is disclosed in U.S. Pat. No. 4,417,759 issued to Victor C. Purrott on Nov. 29, 1983. This design also utilizes a pin member that extends from one of the swivel link mountings. The pin extends outwardly from the swivel link and mounts a plurality of brake discs that extend between the pin and the swivel housing. The pin has a threaded end portion that mounts a nut that is positioned to axially capture the brake discs between a shoulder formed on the pin and the inner surface of the nut. A pair of belleville springs are positioned on each side of the brake discs and provide a compressive force against the brake discs that increases as the nut is tightened against the brake discs. This arrangement is also exposed to structural damage because of is outward positioning with respect to the swivel link. And, since the brake discs are unenclosed, they are subject to contamination by dirt, grease and oil as previously discussed and thus thereby greatly reducing the dampening ability of the mechanism. Finally, the mechanism must be totally disassembled when removal of the swivel link from either mounted component is desired.
The present invention is directed to overcoming one or more of the problems as set forth above.